Method and device for determining the transmittance of a flat glass substrate

ABSTRACT

The invention relates to a method and an associated device for determining the transmittance of a flat-glass substrate ( 40 ) with a measuring device ( 10 ), with which light of at least one light source ( 20 ) is guided from one side of the flat-glass substrate ( 40 ) through the flat-glass substrate ( 40 ) to the opposite side of the flat-glass substrate ( 40 ), where it is captured by at least one receiving unit ( 30 ) and the transmittance of the flat-glass substrate ( 40 ) is determined by means of a comparison between the intensity of the light emitted by the light source ( 20 ) and the light incident upon the receiving unit ( 30 ). The light source is a surface-like diffuse light source ( 20 ), and the receiving unit ( 30 ) comprises at least one spatially resolving receiver ( 31; 31   n ). By evaluating brightness values in the measuring image ( 33; 33′ ) of the spatially resolving receiver ( 31; 31   n ), the transmittance is determined in a spatially resolved manner in a partial surface of the flat-glass substrate ( 40 ), which is covered by the measuring image ( 33; 33′ ).

DESCRIPTION

The invention relates to a method and a device for determining thetransmittance of a flat-glass substrate with a measuring device, withwhich light of at least one light source is guided from one side of theflat-glass substrate through the flat-glass substrate to the oppositeside of the flat-glass substrate, where it is captured by at least onereceiving unit. The transmittance of the flat-glass substrate can thenbe determined by means of a comparison between the intensity of thelight emitted by the light source and the light incident upon thereceiving unit.

In the manufacture of flat glass, a continuous transmittance must beachieved as one of various quality requirements. The thickness of theglass, for example, is another essential parameter. Particularly whenusing flat glass as solar glass, which is a special glass for thermalsolar collectors and photovoltaics, a continuous transmittance has to beobserved. In order to be able to use the incident solar radiationoptimally, as high a transmittance as possible of the supported glasslayer has to be ensured.

In this case, flat glass provided for use as solar glass hasparticularly high requirements with respect to continuous homogeneity.With regard to flat glass as such, a distinction is made between floatglass and rolled glass, the type of glass depending on the productionprocess chosen in each case. Both types of glass are used as solarglass, it being possible to manufacture planar glass in a float process,and structured glass (structural glass) in a rolling process. Typically,both production processes are endless continuous processes in which theglass melt is permanently pulled in a flat manner onto a tin bath orrolled through rotating rollers. The glass is cut to size only afterpassing through a cooling system.

The process of manufacturing solar glass requires a secured, constanttransmittance over the entire production width. Deviations therefromdirectly result in quality problems in processing the glass plates intosolar modules. The structural glass often used for manufacturing solarmodules is cast glass in which the structure is impressed by rotatingrollers. Already the temperature of these rollers and their runningaccuracy affect the glass thickness and transmittance. In addition,parameters such as the inhomogeneity of the glass mixture or impurities,which may lead to different structures, are relevant.

Because the solar cells and the glass cover are irreversibly connectedto each other in photovoltaic module production, for example, a poorglass quality results in an additional amount of rejects of expensivephotovoltaic cells. At present, an insufficient transmittance can onlybe discovered during the output inspection of the finished solar glassproduct, or frequently only in the solar module itself. Therefore, thereject costs are particularly high in this case. In that case, thecoated glass constitutes special waste, and empirically, the reject rateis in the high single-digit percentage range. This great microeconomicand macroeconomic damage and the simultaneous destruction of resources(among other things, silicon in the glass and solar cell production) canonly be reduced by detecting flaws as early as possible during glassproduction.

Measuring methods for verifying the transmittance, by means of which theparameters to be monitored can be randomly verified, were alreadydeveloped. In this case, spectral photometers are commonly used in whichthe wavelength of the incident light beam varies and the transmittedproportion is registered as a function of the wavelength. Optionally,the angular dependence of the incidence of the light is also taken intoaccount in this case. However, such measurements require much effort andare usually only carried out under laboratory conditions, and areunsuitable for use in the production process. Furthermore, they usuallyare point measurements with which it is not possible to inspect theentire surface of a flat-glass substrate.

Particularly in the field of solar glass, the influence of the angle ofincidence of light is considerable because at a smaller angle, largerproportions of light are reflected and absorption increases due to thegreater penetration distances. However, the angle of incidence changesnaturally over the course of the day and the year. Therefore, themanufacturers of solar glass make increased efforts to increase theeffectivity of the solar cells even for flat angles of incidence, bymeans of measures such as anti-reflection coatings and structuredsurfaces. However, the necessary measuring accuracy of measuring methodsused for verifying the transmittance so far does not match thisdevelopment. Particularly the transmittance of structural glass withstructures incorporated into the surface cannot be reliably inspectedwith the known measuring methods.

Therefore, it is the object of the invention to provide a method and adevice for verifying the transmittance of a flat-glass substrate withwhich an extensive transmission measurement can be carried out that canbe integrated into the production process.

In particular, the method and the device are in this case supposed to besuitable for verifying the transmittance of structural glass.

According to the invention, this object is achieved by means of a methodaccording to the independent claim 1. Advantageous embodiments of thismethod are apparent from the dependent claims 2-14. Furthermore, theobject is achieved by means of a device according to the independentclaim 15. Advantageous embodiments of the device are apparent fromclaims 16-20.

It must be noted that the features cited individually in the claims canbe combined with each other in any technologically meaningful manner andrepresent other embodiments of the invention. The description, inparticular in connection with the figures, additionally characterizesand specifies the invention.

The inventive method according to claim 1 is suitable for determiningthe transmittance of a flat-glass substrate with a measuring device,with which light of at least one light source is guided from one side ofthe flat-glass substrate through the flat-glass substrate to theopposite side of the flat-glass substrate, where it is captured by atleast one receiving unit. The transmittance of the flat-glass substrateis determined by means of a comparison between the intensity of thelight emitted by the light source and the light incident upon thereceiving unit. According to the invention, the light source used is asurface-like diffuse light source, and the receiving unit comprises atleast one spatially resolving receiver, between which the flat-glasssubstrate is positioned. By evaluating brightness values in themeasuring image of the spatially resolving receiver, the transmittanceis determined in a spatially resolved manner in a partial surface of theflat-glass substrate, which is covered by the measuring image.

Light can thus be transmitted through a surface of the flat-glasssubstrate in a homogeneous manner by means of the surface-like diffuselight source, and using one or several spatially resolving receivers, itis possible to also determine, by means of this light, the transmittancein a spatially resolved manner within the partial surface of theflat-glass substrate which is covered by the measuring image of therespective receiver.

Preferably, a camera is used as the spatially resolving receiver. Thismay be a camera with a CCD or CMOS chip, for example. With this camera,a measuring image can be prepared that covers, for example, a partialsurface with an order of magnitude of 5×10 cm or 10×20 cm on theflat-glass substrate.

In order to increase the accuracy in the determination of thetransmittance, it is preferably provided that image editing is carriedout in the measuring image of the spatially resolving receiver. Then,the transmittance is determined by means of the brightness values afterthe image editing of the measuring image. Particularly for measuringpoints in the image, the image editing may include an averaging processfor brightness values over several points of at least a portion of themeasuring image. By using a surface-like diffuse light source inaccordance with the invention, a superposition of a plurality of pointsources occurs, thus resulting in a rather more statistical measuredvalue, which is produced by the superposition of the various pointsources and passages through the glass. For the invention, it ispresumed that this measured value should be the same everywhere onaverage.

The portions within the measuring image used for the averaging processshould preferably be chosen to be large enough so that possibledeflections of the light while passing through the flat-glass substratewithin a portion can be captured and included into the averagingprocess. Therefore, when using the method for inspecting structuralglass, a portion of the measuring image used for the averaging process,advantageously, is larger than dimensions of structures in the surfaceof the flat-glass substrate. If, for example, pyramid-like structuresare incorporated into the surface, the portions for the averagingprocess should be larger than the extent of the individual structures.Thus, deflections of the light by the structures can be taken intoaccount by means of suitable image editing. Thus, the invention makes itpossible, in particular, to also determine the transmittance ofstructural glass with sufficient accuracy.

Therefore, the method can be used universally for all flat-glassproduction methods, types of flat glass and applications of flat glass.In particular, it is suitable for verifying the transmittance ofstructural glass. In particular for solar glass production, the detectedtransmittance of float glass should be ≥90%. By structuring and coatingthe surface, an improvement of in each case up to 3% can be achieved.Since the specifications are, as a rule, accurate to 0.1%, the intendedmeasuring accuracy should be ≤0.1%.

In this case, the focus of the invention is, in particular, on ameasurement for quality assurance, i.e. the primary goal of the systemis to ensure a constant quality. Various factors, including, forexample, bubbles in the glass or an increased absorption in the glassitself, may be the cause for a reduced transmission. Increasedabsorption may be caused by impurities in the glass. In this case,iron-containing admixtures are typical, but there are also other mineralimpurities that are capable of increasing the absorption in the glass.The thickness of the glass also has an influence on the transmittance;however, this influence is relatively small.

Impurities in the glass, which cause a reduction in transmittance due toabsorption, are located for quality assurance. Based on knowntransmission spectra of glass, this goal can be achieved by measuringthe transmittance for individual selected wavelengths, for example,which are realized by means of a surface-like diffuse light source withmonochromatic or narrow-band light sources. The wavelengths chosen arepreferably those that are selectively absorbed by potential impuritiesin the glass. The receiving unit is in that case tuned to the chosenwavelengths. As an alternative or addition to measurements withmonochromatic of different wavelengths, a measurement with white lightmay also be carried out.

In the process, an absolute value for the transmittance may bedetermined. For a quality assurance process, however, it may also besufficient to merely determine the deviation of the transmission from atarget value and to thus verify the transmittance. In one embodiment ofthe invention, a verification of the transmittance is thus carried outby means of a comparison of a transmittance determined by the measuringdevice with a target transmittance. Basically, a target-actualcomparison of the transmittance is then carried out.

The method may be carried out with a stationary measuring device intowhich a flat-glass substrate is temporarily introduced. Given anappropriate extent of the light source and a receiving unit with, forexample, a plurality of receivers, the transmittance can thus beverified within a larger area of a flat-glass substrate. In a preferredembodiment of the invention, however, the flat-glass substrate and themeasuring device are continuously moved relative to one another in sucha manner that over the course of this movement, the receiving unitcaptures the light passage through different partial areas of theflat-glass substrate. In this manner, a flat-glass substrate can beinspected in its entirety if a movably configured measuring device scansthe entire surface of a substrate, for example. In order to avoidmovable components of a measuring device, however, it was found to beadvantageous if the latter is substantially stationary and theflat-glass substrate is moved relative to the measuring device, in orderto also scan the entire surface of a substrate in this manner.

In one embodiment of the invention, in order to integrate the methodinto the production process of flat-glass production in an advantageousmanner, it is provided, in particular, that the measuring device isstationary, whereas the flat-glass substrate is continuously movedthrough between the light source and the receiving unit. In this case,the measuring device can be configured as a kind of portal through whicha continuously produced glass sheet is permanently guided between thelight source and the receiving unit.

Preferably, the flat-glass substrate has an extent with the width Btransverse to the direction of the relative movement between themeasuring device and the flat-glass substrate, with the light sourcetransmitting light through the flat-glass substrate over the entirewidth B. A correspondingly configured receiving unit can then capturethe light passing through a strip through the flat-glass substrate thatextends over the entire width B of the flat-glass substrate. A type ofline scan can be performed in this manner, whereby a full-surfaceinspection of a glass sheet can be carried out continuously in aflat-glass substrate moved constantly through the measuring device.

In particular, the receiving unit has several receivers (cameras) forthis purpose, which are configured for capturing light of the lightsource and whose measuring images are combined into an image of a planararea of the flat-glass substrate by an image processing unit. In thismanner, a greater partial surface of the flat-glass substrate can beinspected. For a line scan, the receivers of the receiving unit aredisposed next to one another, for example, and their measuring imagesare combined into an image of a strip of the flat-glass substrateextending over the entire width B. In any case, it is advantageous hereif in this case, the measuring images of the receivers overlap in someareas. Thereby, the accuracy of the determination of the transmittancecan be increased if, for example, brightness values of one and the samepoint in two overlapping images are compared to each other and thusverified, if possible. In the case of deviating brightness values for asingle point, an averaging process may also be carried out. Theoverlapping measuring images may also be used for a self-calibration ofthe measuring system with several receivers.

On the whole, the invention makes it possible to carry out atransmission measurement in an extensive manner within a continuousproduction process for flat glass, and not as a single measurement onthe cut goods. However, the single measurement on cut goods is notexcluded; rather, the invention entails the aforementioned advantageswith respect to an extensive verification of the transmittance also forthis embodiment.

In particular, it is also possible to measure the transmission directlyin the production process and thus immediately intervene in the runningmanufacturing process. Thus, the method makes it possible to conform tothe high requirements for solar glass, for example, already during theproduction. The reject rate of coated glass, and subsequently inphotovoltaic and collector modules, is reduced in a sustained manner bya continuous transmission measurement. It is therefore an importantecological and economic advantage that the amount of rejects inproduction and in further processing (coating or preparation ofcomposite glass) can be significantly reduced. This results inconsiderable savings in reject and disposal costs.

In this case, a use under production conditions is possible, whichparticularly includes high temperatures and extraneous light (e.g.lighting, industrial trucks). By using modulated light, the influence ofextraneous light can be eliminated, for example.

In one embodiment of the invention, it is additionally provided thatlight of the light source is captured by the receiving unit in at leastone area next to the flat-glass substrate without passing through theflat-glass substrate. For this purpose, the light source may beconfigured to be wider than the flat-glass substrate to be inspected, sothat areas to the left and to the right of a glass sheet, for example,are also illuminated. In that case, the receiving unit is configured insuch a way that it is capable of capturing also this light. Thus, a zeromeasurement without a medium, which corresponds to a transmission of100%, can be carried out in the border area of the flat-glass substrate.This zero measurement reflects changes in the external conditions thatmay occur during continuous operation of the measuring device and whichmust be taken into account during calibration. The light of the lightsource captured by the receiving unit without passing through theflat-glass substrate can therefore, in particular, be used as areference value for the calibration of the measuring device.

Preferably, the calibration takes place in the form of aself-calibration of the system. In one embodiment of the invention, thiscan also take place continuously during the verification of thetransmittance, because the zero measurement next to the substrate can becarried out constantly. Thus, the production process of a glass sheetneed not be interrupted; rather, the system automatically calibratesitself constantly. For example, the measuring image of at least oneouter receiver can be used for this purpose, which reproduces a borderarea of the flat-glass substrate but simultaneously also covers an areanext to the flat-glass substrate in which light from the light source isincident on the receiver without passing through the glass. Thus, areference value with a defined calibration value for a transmission of100% is available to this outer receiver from a zero measurement, withwhich this receiver can be calibrated. This also applies to a second,opposite outer receiver at the other edge of the glass sheet, which mayalso be calibrated in this manner. Based on these two outer receivers,the measuring system can then be calibrated by receivers positionedfurther inward in a step-by-step alignment with the respectivelyadjacent measuring system. The overlapping of the measuring images ofthe individual receivers already mentioned above is advantageous forthis alignment. In addition to its own data, each receiver thus also hasat its disposal data for an overlapping area from at least one adjacentcamera that can be used for an alignment and a calibration. Thus, thereference value determined by one of the two outer receivers in a zeromeasurement can be incorporated, step by step, into each alignment. Asuitable self-calibration of the system can be realized in this manner,which makes the measuring device particularly suitable for use directlyin the production process, with variable and extreme environmentalinfluences.

The invention also includes an associated device for determining thetransmittance of a flat-glass substrate, comprising a measuring device,with which light of at least one light source can be guided from oneside of the flat-glass substrate through the flat-glass substrate to theopposite side of the flat-glass substrate, where it is captured by atleast one receiving unit. The device has a data processing unit inconnection with the light source and the receiving unit, which isconfigured for determining the transmittance of the flat-glass substrateby means of a comparison between the intensity of the light emitted bythe light source and the light incident upon the receiving unit. In thiscase, the light source is in this case a surface-like diffuse lightsource, and the receiving unit comprises at least one spatiallyresolving receiver, between which the flat-glass substrate can bepositioned. The data processing unit is further configured fordetermining the transmittance in a spatially resolved manner in apartial surface of the flat-glass substrate, which is covered by themeasuring image, by evaluating brightness values in the measuring imageof the spatially resolving receiver.

Preferably, the device has means for continuously moving the flat-glasssubstrate and the measuring device relative to one another, wherein thelight passage through different partial areas of the flat-glasssubstrate can be captured with the receiving unit over the course ofthis movement. Thus, the device is configured for verifying thetransmittance within a larger surface of the flat-glass substrate. Forexample, the measuring device can in this case be stationary, whereasthe flat-glass substrate is continuously moved through between the lightsource and the receiving unit. As was already explained above, themeasuring device can thus, in particular, be integrated into aproduction process, with a continuously produced glass sheet passingthrough the measuring device in a continuous strip.

In this case, the dimensions of the device are preferably chosen in sucha way that the flat-glass substrate, transverse to the direction of therelative movement between the measuring device and the flat-glasssubstrate, has an extent with the width B that is less than the width bof the light source in the same direction. Typical widths for flat-glasssubstrates are in the order of magnitude of about 3.5 to 4 m, and thelight source may protrude over them in each case by 10 cm on twoopposite sides, for example.

In order to carry out a line scan, the receiving unit preferably hasseveral spatially resolving receivers, such as cameras, disposedside-by-side. In that case, the data processing unit is configured forcombining the measuring images of the receivers into a larger image of aplanar area of the flat-glass substrate. Further, the data processingunit is configured for the above-mentioned calibration of the measuringdevice by means of light of the light source captured by the receivingunit in an area next to the flat-glass substrate without passing throughthe flat-glass substrate. As a whole, the device, and in particular thedata processing unit, are therefore configured for carrying out one ormore embodiments of the method according to the invention.

Other advantages, special features and expedient further developments ofthe invention are apparent from the dependent claims and the followingpresentation of preferred embodiments with reference to theillustrations.

IN THE DRAWINGS

FIG. 1: shows a schematic view of the components of a system with ameasuring device for carrying out the method according to the invention;and

FIG. 2: shows a schematic cross-section through a light source, aflat-glass substrate and a receiving unit.

The invention utilizes the principle of the quantitative measurement ofthe transmittance as a ratio of the intensity of a light beam after itspassage through the medium to be measured to that of the incident lightbeam. For this purpose, it is necessary to know or measure the intensitybefore and after passing through the obstacle in a measuring device. Thefundamental components of an exemplary embodiment of a system with sucha measuring device are explained below with reference to the schematicillustrations of FIGS. 1 and 2.

In this case, the measuring device 10 of the exemplary embodiment ofFIG. 1 comprises a surface-like diffuse light source 20. The latter mayalso be formed by an LED panel with a plurality of light-emitting diodes21 under a light-transmissive, plate-shaped diffuser 22 (also see FIG.2). The diffuser 22 may be configured as a film, for instance. Such apanel substantially consists of a circuit board on which light-emittingdiodes (LEDs), for example of the SMD type, are mounted in atwo-dimensional array. In particular, the LEDs are white-light diodes. Awhite-light LED may be realized, for example, by a blue LED with aconversion layer based on phosphorescence. These white-light diodes canbe supplemented with LEDs generating monochromatic light of differentwavelengths. For example, yellow, green, red and/or blue LEDs may bemounted on the same panel. The selection of wavelengths of the LEDs inthis case particularly depends on which spectral range is consideredrelevant for finding a potential impurity.

Within a light wall 20 configured as a light matrix or illuminatingmatrix, the LEDs are disposed in uniform rows, for example. However, thelight-emitting diodes may also be disposed offset relative to oneanother, or in other special patterns. In particular, an arrangement inhexagonal grids has proved to be useful. The distance between theindividual LEDs may suitably be in the order of magnitude of 1 to 4 cm,in particular about 2 cm. An LED panel may in this case comprise about1000 LEDs, for example. Preferably, each LED is individuallycontrollable, or groups of LEDs can be controlled separately from oneanother. Preferably, the intensity of the light source 20 is flexiblyadjustable, which may take place via a hardware-based current regulationof the LEDs, for example. Advantageously, however, a software-basedpulse-width modulation is used (PWM control).

Furthermore, both patterns varying over time and light with differentdiscrete spectra—corresponding to the mounting of different LEDs—may beproduced by means of the possible individual control of the LEDs. Inthis case, a selection of the LEDs best suited for the specificmeasurement purpose may take place.

The surface-like light source 20 configured in this manner is disposedabove a flat-glass substrate 40, for instance, so that the glass ishomogeneously illuminated from above. As a further part of the measuringdevice 10, a receiving unit 30, which comprises at least one spatiallyresolving receiver in the form of, for example, a camera, is locatedunderneath the flat-glass substrate 40. However, the arrangement of thesurface-like diffuse light source 20 and the receiving unit 30 may alsobe reversed, so that the light source 20 is located underneath and thereceiving unit 30 above the glass to be inspected.

Preferably, several cameras 31, . . . , 31 n are disposed side-by-side,as is also apparent from the cross-section of FIG. 2. In this case, theglass sheet 40 has an extent of the width B transverse to the direction11 of its movement. By means of a receiving unit 30 with several cameras31, . . . , 31 n disposed side-by-side, a strip-shaped image of thepassing flat-glass substrate 40 over its entire width B can be preparedfrom the images of these cameras. Thus, the receiving unit 30 is sowide, or has so many receivers disposed side-by-side, that the glasssheet 40 can be captured in its entire width B. Accordingly, the lightsource 20 is also configured in such a way that it is capable ofuniformly transmitting light through the entire width B of theflat-glass substrate 40. In this way, a line scan of the glass sheet 40over its entire width B can be generated by verifying the transmittancein a strip extending over the entire width of the flat-glass substrate.Preferably, the measuring images in the plane of the glass sheet, whichcan be captured by one camera each, overlap in this case. By way ofexample, FIG. 2 shows two overlapping areas/measuring images 33 and 33′of two cameras situated next to one another.

For example, the flat-glass substrate 40 is a glass sheet moving in thedirection of the arrow 11 through the measuring device 10. Thus, themeasuring device 10 with the light source 20 and the receiving unit 30is stationary in this exemplary embodiment, whereas the glass sheet 40to be inspected moves relative to the measuring device 10. In this case,the glass is not yet cut into glass plates, but is a continuouslyproduced endless sheet running through the measuring device 10, as it istypically produced in float-glass or structural-glass plants. However,the invention may also be used in glass panes already cut to size.

In this case, the measuring device 10 may also be configured so as to bemovable, and may be moved across different areas of a flat-glasssubstrate, for example, which is then stationary or is also movedsimultaneously. Both in the stationary and the movable version, themeasuring device 10 can be configured in the form of a portal with anupper and a lower side, between which the flat-glass substrate to beinspected can be positioned.

The light source 20 is operated in a pulsed manner, with the cameras ofthe receiving unit 30 being synchronized to this quasi-stroboscopicoperation. In this case, the flash rate of the light source 20 is, forexample, in the range of a few 10 to 100 Hertz, and the flash durationin the microsecond range. Typically, the shutter times of the camerasare longer than the flash duration of the light source. At least forthis synchronization, the comparison of emitted and received lightintensity and for the determination of the transmittance, the lightsource 20 and the receiving unit 30 are connected to a data processingunit 50. In particular, the data processing unit 50 is configured fordetermining the transmittance of a partial surface of the glass sheet ina spatially resolved manner and preferably also verify or assess itbased on the target value, based on the intensity of the light of thelight source 20 and brightness values for the light received by thespatially resolving receivers 31, . . . , 31 n.

The data processing unit 50 further comprises an image processing unit51, which is preferably capable of preparing an image of a capturedpartial surface of the flat-glass substrate from the images of thecameras. The image processing unit 51 further comprises means forcarrying out image editing based on the measuring images of theindividual receivers of the receiving unit, which, in particular, alsoincludes an averaging process.

In turn, the data processing unit 50 may be connected with a controlunit 60 for the production process within which the flat-glass substrateis produced. When the transmittance of the substrate deviates from atarget value, the data processing unit 50 may notify the control unit 60of this, which then may register the substrate as unsuitable and/orchange parameters of the production process in order to bring thetransmittance back into the desired range.

In this case, the width b of the light source 20 preferably exceeds thewidth B of the flat-glass substrate 40. This also applies for the widthof the receiving unit 30 or the maximum area that can be imaged by thecameras of the receiving unit 30. In this case, as can be seen in FIG.2, an outer area 32 is respectively provided to the left and to theright next to the glass sheet 40, within which light of the light source20 can be incident upon an outer camera 31 n of the receiving unit 30without passing through the substrate 40. These areas without lightpassage through the flat-glass substrate may be used as zero measurementfor a calibration of the measuring device 10.

The suitable distance between the LED panel 20 and the plate-shapeddiffuser 22 is strongly dependent upon the selected type of diffuser andits properties. For example, it may be in the order of magnitude of 5 to20 cm, in particular about 10 cm. A distance between the diffuser andthe flat-glass substrate of 15 to 45 cm, in particular about 30 cm, hasproved to be advantageous, whereas the distance between the receivingunit and the flat-glass substrate as an object distance may be in theorder of magnitude of 70 to 150 cm, in particular about 110 cm. Thisdistance is strongly dependent upon the camera sensor and the lens ofthe camera. Further, the distances between the flat-glass substrate andthe components of the measuring device are relevant due to the fact thata measurement is intended preferably on the still-warm substrate, inparticular even while the production process is running. Thus, themeasuring device is disposed opposite to a glass strip having atemperature of at least 40 to 45° C. However, its temperature may alsobe significantly above 100° C. The hardware must be adapted to theseconditions.

In addition to the above-mentioned hardware, the invention also includesthe associated software. The latter is primarily composed of thesoftware for controlling the light sources and receivers, software forevaluating the determined data, and an adapted calibration algorithm.The data processing unit 50 processes the measurement data and is ableto forward the corresponding information to the control unit 60 of theproduction plant or to quality assurance, for example. In this case, thereal time behavior of the system is of particular significance, and thehigh data rates produced during the use of cameras need to be optimized.The very substantial data volume captured by the cameras may be reduced,for example, by pre-evaluation. In particular, this makes it possible tobe able to transmit the available measurement data via standardinterfaces (LAN, WLAN, CAN bus, etc.) to processing systems without theoccurrence of bandwidth issues.

The basic measuring process used for determining or verifying thetransmittance of the flat-glass substrate 40 is based on the knownwavelength of the light of the light source 20 and the known spectrum ofthe flat-glass substrate. The glass sheet 40 constitutes an obstacle forthe propagation of the light of the light source 20. The transmittance τdescribes the proportion of the transmitted luminous flux relative tothe incident luminous flux captured by a camera of the receiving unit30. If I_(o) is the intensity of the incident light, and I_(p) is theintensity of the light behind the glass sheet, then τ=I_(p)/I₀.

The losses are the result of reflection on the surface and absorptionwhen passing through the glass sheet. In this case, absorption and thusalso transmission fundamentally depend on the composition of the glassand its thickness. As a rule, the light sources that play a role ineveryday life (sun) are not monochromatic but contain light withdifferent wavelengths. In this case, the transmittance iswavelength-dependent.

In order to determine the transmittance in a continuous spectrum, alight source with the corresponding spectrum is usually required. Usinga monochromator, it is possible to adjust the spectrum and determine thetransmission for each wavelength. However, the limited temporalresolution is disadvantageous in this case. Furthermore, such a processtakes too long to integrate it into an inline inspection of a glassstrip passing through continuously. Therefore, it may be advantageous tolimit the relevant part of the spectrum with respect to the intendedapplication of the glass. For a robust method for use in production, ithas proved to be advantageous, for example, if a limitation to adiscrete spectrum is provided which can be realized by mono-chromaticlight sources 20 within the light wall. This may be sufficient forquality assurance. The limitation to selected wavelengths is alsoadvantageous in that interfering influences, particularly due toextraneous light, are easier to eliminate.

In this case, it is necessary to define corresponding representativewavelengths, taking into account the solar spectrum and the spectralcharacteristics of the various solar cell types, and realize them withthe LEDs of the light source 20. The receiving unit 30 is then tuned tothese wavelengths. The refresh rate is in this case determined for aquasi-continuous measurement depending on the travel speed of the glasssheet.

In addition or as an alternative, it is also possible to carry out awhite-light measurement with white-light diodes of the light source 20.Thus, impurities can be found for all included wavelengths which couldcause a reduction of the transmittance by means of selective absorption.

With the method according to the invention, the transmittance of aflat-glass substrate 40 can be determined over its entire surface,preferably in a spatially resolved manner. For mere quality assurance,it may possibly suffice if an absolute measurement of the transmittanceis replaced by a comparison measurement (deviation from a target). Inthat case, it is sufficient to generate the reference values by means ofmaterial with the desired properties.

In addition to a homogenous planar illumination of the glass sheet bythe surface-like light source 20, there is also the option of generatinginspection patterns by individually controlling the LEDs. In thismanner, the device can be extended also with respect to defectrecognition. Together with the targeted triggering of camera shots,different measurements can then be carried out quasi-simultaneously.Using hardware triggers, short light flashes with various properties canbe produced. Mention may be made, for example, of a homogeneousillumination, patterns emphasizing individual LEDs, or gray gradientsproduced by different flash durations of the individual LEDs.

A calibrated measuring system is a fundamental requirement for an exactand reproducible measurement. Therefore, a calibration method ispreferably used which can be integrated into the inline measuringmethod, because flat-glass production is an interruption-free,continuous process. Therefore, the process cannot be interrupted for acalibration of the measuring device. In this case, an intensityregulation of the LED spotlights by means of PWM instead of theconventional current regulation provides a means for being able toarbitrarily adjust and re-adjust the brightness of the light source 20if required. Furthermore, with PWM, age-related reductions of the lightintensity can be compensated individually for each LED in the onlinecalibration method, and a high level of long-term stability of the lightsource can be ensured.

With a typical sheet width B of 3.5 to 4 meters and measuring equipmentboth on the upper and the lower side, it is difficult to calibrate themeasuring devices in an offline process because this entailsconsiderable conversion effort. In contrast, movable parts, which wouldalso be disadvantageous, would be required for an automated calibrationprocess.

Therefore, in one embodiment, the invention uses an approach for acalibration that gets along without any movable parts. In this case, allof the light sources and cameras used are calibrated in a step-by-stepalignment with the neighboring system, wherein the respective outercameras, due to zero measurements next to the glass sheet, are providedwith reference values with defined calibration values. Thus, it ispossible to carry out an alignment with a reference system as well asassess the quality of the calibration. With the reference systems onboth sides, a faulty calibration that may be produced by instantaneouschanges of the glass properties can be excluded.

LIST OF REFERENCE NUMERALS

-   10 Measuring device-   11 Direction of relative movement, direction of movement of glass    sheet-   20 Light source, light wall, LED panel-   21 Light source, light-emitting diode, LED-   22 Diffuser-   30 Receiving unit-   31, 31 n Receiver, camera-   32 Outer measurement area-   33, 33′ Measuring image-   40 Flat-glass substrate, glass sheet-   50 Data processing unit-   51 Image processing unit-   60 Control unit of a production plant

1. A method for determining the transmittance of a flat-glass substratewith a measuring device, with which light of at least one light sourceis guided from one side of the flat-glass substrate through theflat-glass substrate to the opposite side of the flat-glass substrate,where it is captured by at least one receiving unit and thetransmittance of the flat-glass substrate is determined by means of acomparison between the intensity of the light emitted by the lightsource and the light incident upon the receiving unit, wherein the lightsource is a surface-like diffuse light source, and the receiving unitcomprises at least one spatially resolving receiver, between which theflat-glass substrate is positioned, and the transmittance is determinedin a spatially resolved manner in a partial surface of the flat-glasssubstrate, which is covered by the measuring image, by an evaluation ofbrightness values in the measuring image of the spatially resolvingreceiver.
 2. The method of claim 1, wherein the flat-glass substrate andthe measuring device are continuously moved relative to one another insuch a manner that over the course of this movement, the receiving unitcaptures the light passage through different partial areas of theflat-glass substrate.
 3. The method of claim 2, wherein the measuringdevice is stationary, whereas the flat-glass substrate is continuouslymoved through between the light source and the receiving unit.
 4. Themethod of claim 1, characterized in that wherein in the measuring imageof the spatially resolving receiver, an image editing is carried outwith includes an averaging process for brightness values over severalpoints of at least a portion of the measuring image, and thetransmittance is determined after the image editing of the measuringimage.
 5. The method of claim 4, wherein the portion of the measuringimage used for the averaging process is larger than dimensions ofstructures in the surface of the flat-glass substrate.
 6. The method ofclaim 1, wherein a verification of the transmittance is carried out bymeans of a comparison of a transmittance determined by the measuringdevice with a target transmittance.
 7. The method of claim 2, whereinthe flat-glass substrate has an extent with the width B transverse tothe direction of the relative movement between the measuring device andthe flat-glass substrate, with the light source transmitting lightthrough the flat-glass substrate over the entire width B and thereceiving unit capturing light passing through a strip through theflat-glass substrate extending over the entire width B of the flat-glasssubstrate.
 8. The method of claim 1, wherein the receiving unit hasseveral receivers, which are configured for capturing light of the lightsource and whose measuring images are combined into an image of a planararea of the flat-glass substrate by an image processing unit.
 9. Themethod of claim 8, wherein the receivers are disposed next to oneanother, and their measuring images are combined into an image of astrip of the flat-glass substrate extending over the entire width B. 10.The method of claim 8, wherein the measuring images of the receiversoverlap in some areas.
 11. The method of claim 1, wherein light of thelight source is captured by the receiving unit in at least one area nextto the flat-glass substrate without passing through the flat-glasssubstrate.
 12. The method of claim 11, wherein the light of the lightsource captured by the receiving unit without passing through theflat-glass substrate is used for defining a reference value for thecalibration of the measuring device.
 13. The method of claim 13, whereina measuring image of at least one outer receiver, which reproduces aborder area of the flat-glass substrate, also includes an area next tothe flat-glass substrate via which this outer receiver obtains areference value with a defined calibration value and the measuringsystem of further receivers is calibrated in a step-by-step alignmentwith the respectively adjacent measuring system.
 14. The method of claim13, wherein the calibration of the measuring device takes placecontinuously during the verification of the transmittance.
 15. A devicefor determining the transmittance of a flat-glass substrate, comprisinga measuring device, with which light of at least one light source can beguided from one side of the flat-glass substrate through the flat-glasssubstrate to the opposite side of the flat-glass substrate, where it iscaptured by at least one receiving unit, and the device has a dataprocessing unit in connection with the light source and the receivingunit, which is configured to determine the transmittance of theflat-glass substrate by means of a comparison between the intensity ofthe light emitted by the light source and the light incident upon thereceiving unit, wherein the light source is a surface-like diffuse lightsource, and the receiving unit comprises at least one spatiallyresolving receiver, between which the flat-glass substrate can bepositioned, and the data processing unit is further configured fordetermining the transmittance in a spatially resolved manner in apartial surface of the flat-glass substrate, which is covered by themeasuring image, by an evaluation of brightness values in the measuringimage of the spatially resolving receiver.
 16. The device of claim 15,wherein the device has means for continuously moving the flat-glasssubstrate and the measuring device relative to one another, wherein thelight passage through different partial areas of the flat-glasssubstrate can be captured with the receiving unit over the course ofthis movement.
 17. The device of claim 16, wherein the measuring deviceis stationary, whereas the flat-glass substrate is continuously movedthrough between the light source and the receiving unit.
 18. The deviceof claim 15, wherein the flat-glass substrate, transverse to thedirection of the relative movement between the measuring device and theflat-glass substrate, has an extent with the width B that is less thanthe width b of the light source in the same direction.
 19. The device ofclaim 15, wherein the receiving unit has several spatially resolvingreceivers disposed side-by-side.
 20. (canceled)